The manganese aggregate in photosystem II (PSII) is arguably one of the most important metal sites in biology, as it is the source of the O2 required for aerobic life. A number of polynuclear manganese complexes have been prepared which exhibit structural and spectroscopic features relevant to PSII. Raman spectroscopy is being used to study the core structure (Mn4O6)4+ of these complexes to determine the presence of an O-O bond. Formation of an O-O bond between terminal or bridging oxo-ligands prior to release of O2 has been proposed as one of the mechanisms for water oxidation in PSII.